1. Field of the Invention
The invention is in the field of optical signal transmission and techniques for polarization-insensitive coherent signal detection. More particularly, the invention relates to an improvement in a system and a method of optical signal transmission with polarization-insensitive coherent detection and frequency stabilization at the receiving end.
2. Prior art
A method of achieving reliable coherent signal reception is to use a polarization-insensitive detection. One of the standard techniques used for this purpose is the one in which so-called `data-induced polarization switching` (DIPS) is employed. Such a technique is disclosed, for example, in reference [1]. In this technique, the "zeros" and the "ones" of binary-coded data are transmitted as optical signals having different, mutually orthogonal polarization. This achieves the result that, in a coherent receiver, a signal, at least either only the "zeros" or only the "ones", but generally both, is detected for any polarization setting whatsoever. Such a method of polarization switching can be achieved, as is also disclosed in reference [1] by causing a signal which is FSK-modulated in accordance with the binary-coded data to propagate through a birefringent fiber. In this situation, the length of the fiber is determined by the frequency swing used in the modulation. In coherent transmission systems in which, to achieve a polarization-insensitive signal transmission, DIPS is applied to FSK-modulated signals, the received light signal is coupled at the receiving end to a light signal originating from a local oscillator and picked up by a detector. Two operations are performed on the detected electrical signal downstream of the detector. The first operation relates to a demodulation in which the original data is recovered from the electrical signal. In the second operation; a control signal which is fed back via a frequency control loop to the local oscillator for the purpose of automatic frequency control (AFC) is derived from the electrical signal. Said control signal is proportional to the difference between a desired set frequency, the so-called intermediate frequency (IF) and a frequency interval (IF'), derived from the electrical signal, between the signal frequencies of the local oscillator and the received signal. In an AFC control as disclosed, for example, in references [2] and [3], a frequency discriminator is used to derive the control signal. Such a discriminator has a transfer function having a number of passages through zero which are chosen in such a way that they can coincide with the frequency signal peaks of the logic values "zero" and "one", such as those which may occur in the received FSK-modulated signal. As a result, even if the `ones` or the `zeros` are missing, the discriminator is able to generate an electrical signal which is proportional to the frequency interval IF'. As a result of the operation, a control voltage is obtained with which the local oscillator can be adjusted in such a way that a frequency deviation between IF' and IF is corrected both for magnitude and for sign. Such an AFC control achieves a stable frequency tuning since the control continues to operate even if one of the frequency signal peaks, either that of the "ones" or that of the "zeros" were to be missing at any random polarization setting. A drawback is, however, that, if both `zeros` and `ones` are not already present in the detected signal during a start-up procedure in which the frequency of the local oscillator is set to the vicinity of the IF and the AFC control loop is then closed, such a stable frequency tuning cannot be brought about. If only one frequency signal peak is detected during start-up, it is not possible, after all, to discern on what side thereof the IF is.